AC/Delco is a great plug and is the OE plug. I would install the exact same plug that it came with.
What is the spark plug gap setting for a 89 cadillac deville?
Distributor cap, rotor, plug wires or spark plugs could do that. Time for a tune up.
Spark plug gap is .028
.45
0.045
.035
Most of these were .045 but you should check with a AC spark plug application book.
Under the spark plug wires in the cylinder head, on top of the engine.
.035
gap it to 44
.035 inch
.060" is what the parts stores say.